Photoemission study of the interaction of a reduced thin film SnO2 with oxygen

doi:10.1016/S0039-6028(99)00044-8

Surface Science

Volumes 433–435, 2 August 1999, Pages 226-229

https://doi.org/10.1016/S0039-6028(99)00044-8 Get rights and content

Abstract

The interaction of oxygen with a reduced thin film SnO₂ has been studied with X-ray photoelectron spectroscopy. Three surface oxygen species (O⁻, O²⁻₂, O⁻₂) were observed in the valence band spectra after O₂ exposure onto the reduced SnO₂ thin film. O⁻ and O²⁻₂ were assumed to be adsorbed on the Sn²⁺ sites and O⁻₂ ions were on the Sn⁴⁺ sites. These oxygen species were desorbed in the order O⁻, O²⁻₂ (>473 K) and O⁻₂ (>673 K).

Introduction

The surface electronic states of tin oxide have been intensively studied experimentally and theoretically [1], [2], [3], [4], [5], [6]. Very recently, we theoretically calculated the stability of oxygen anions on a reduced SnO₂ (110) crystal surface using a point-charge model [7]. Oxygen species on the SnO₂ surface were stabilized in the order of O²⁻₂, O²⁻, O⁻ and O⁻₂. These results indicate the low concentration of O⁻ and O⁻₂ but O²⁻₂ is expected to have a somewhat large concentration on the surface. Very few experimental studies of adsorbed oxygen species on reduced SnO₂ surfaces have been examined [3], [8], [9]. Shen et al. [3] studied the interaction of O₂ with SnO₂ (110) and reported that the formation of superoxo O⁻₂ was suggested and this type of oxygen species desorbed from Sn²⁺ atoms around 250 K. Mizokawa and Nakamura [9] reported that this O⁻₂-type species desorbed around 420 K from polycrystalline SnO₂. The formation of other oxygen species like O⁻ and O²⁻₂ on SnO₂ surfaces is controversial [10].

In this paper, we examine O₂ adsorption onto a reduced SnO₂ thin film at various temperatures up to 773 K. The chemisorbed states were examined on the X-ray photoelectron spectroscopy (XPS) spectra for the core levels and the valence band (VB) level. The chemical shift from Sn²⁺ to Sn⁴⁺ also was examined after oxygen exposure at each temperature in the VB spectra.

Section snippets

Experimental procedure

Tin oxide thin films were deposited on sapphire substrates by the reactive RF magnetron sputtering method using an SPF-430 apparatus produced by Nichiden Anelva. The sputtering target was sintered SnO₂ (99.9% purity). We selected the sputtering conditions for preparing a dense texture. The details of the sputtering conditions are in our previous paper [6].

The details of XPS experimental procedure are also in our previous papers [11], [12]. A monochromatized Al Kα (1486.6 eV) X-ray source was used

Results and discussion

We measured the etched SnO₂ thin film in the VB region as shown in Fig. 1. This spectral pattern shows that the surface state of the etched sample is mainly SnO₂ [14]. However, the presence of the band gap emission shows the formation of Sn²⁺ ions, as reported on the reduced tin oxides [11], [12]. We considered the bridging oxygen was removed from the topmost layer and Sn²⁺ ions were produced. In order to obtain more quantitative information of the compositional changes of the surface layers,

Conclusions

Three surface oxygen species (O⁻, O²⁻₂, O⁻₂) were observed in the VB spectra after O₂ exposure onto the reduced SnO₂. These oxygen species were desorbed in the order of O⁻, O²⁻₂ (>473 K) and O⁻₂ (>673 K) as expected from our previous theoretical calculations about the stability of these oxygen ions on the reduced SnO₂ [7]. O⁻ and O²⁻₂ ions are assumed to be adsorbed on Sn²⁺ sites and O⁻₂ ions are at low coordinated Sn⁴⁺ sites.

Acknowledgements

This study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). One of us (K.T.) is grateful to Professor M.W. Roberts at Cardiff University for his discussions about oxygen species.

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Photoemission study of the interaction of a reduced thin film SnO2 with oxygen

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgements

Vacuum

Vacuum

Appl. Surf. Sci.

Adv. Catal.

J. Mol. Catal. A:

Surf. Sci.

Appl. Surf. Sci.

Surf. Sci.

Appl. Surf. Sci.

Surf. Sci. Lett.

Phys. Rev. B

Photoemission study of the interaction of a reduced thin film SnO₂ with oxygen